New Physics: Sae Mulli, Vol. 65, No. 8, August 2015, pp. 715∼720
DOI: 10.3938/NPSM.65.715
Structural and Electrical Properties of Bi0.5 Na0.5 TiO3 Templates Produced by Topochemical Microcrystal Conversion Method Ali Hussin · Adnan Maqbool · Rizwan Ahmed Malik · Min Su Kim · Tae-Kwon Song · Myong-Ho Kim∗ School of Advanced Materials Engineering, Changwon National University, Changwon 641-773, Korea
Arif Zaman Department of Physics, Abdul Wali Khan University Mardan, Khyber Pakhtunkhwa, Pakistan
Won-Jeong Kim Department of Physics, Changwon National University, Changwon 641-773, Korea (Received 24 April 2015 : revised 5 June 2015 : accepted 5 June 2015)
Bi0.5 Na0.5 TiO3 (BNT) templates with a single phase perovskite structure were produced from a plate-like precursor particles of bismuth-layer-structured ferroelectric Bi4.5 Na0.5 Ti4 O15 (BNT4) through a topochemical microcrystal conversion process. First, the plate-like BNT4 precursor particles were prepared via molten salt synthesis. The layered structure BNT4 transformed into a single phase perovskite BNT templates after its topochemical reaction with the complementary Na2 CO3 and TiO2 reactants at 950 ◦ C for 4 h in a NaCl flux. The as synthesized BNT templates exhibited large grains (range from 10 to 15 µm), had plate-like morphology and exhibits a single-phase perovskite structure with a pseudo-cubic symmetry. Furthermore, the temperature dependences of dielectric constant and loss at different frequencies showed a relaxor behavior, and polarization versus electric field curves exhibited a typical ferroelectric response. PACS numbers: 61.05.cp, 77.22.Ej Keywords: BNT template, Molten salt synthesis, Topochemical microcrystal conversion, Lead-free ceramics
I. INTRODUCTION
and its solid solutions are intensively studied for the improvement of their electromechanical properties [5–10].
Environmental problems of lead-based piezoelectric
Templated grain growth (TGG) technique has been
materials stimulate the development of high performance
widely used to produce textured piezoelectric ceram-
lead-free counter parts [1–3]. Recently, much research
ics with enhanced electromechanical properties [11–13].
has been carried out on Bi-containing perovskite ma-
Anisotropic template particles play a crucial role in
terials because of its similar electronic structure with
TGG processes. For Bi0.5 Na0.5 TiO3 (BNT)-based tex-
Pb2+ ions which has a 6s lone pair configuration, that
tured ceramics, bismuth layer-structured ferroelectric
is considered highly beneficial for superior piezoelec-
(BLSF) Bi4 Ti3 O12 (BIT) templates or plate-like SrTiO3
tric response [4]. Among, Bi-based perovskite ceramics,
(ST) templates were frequently utilized for their grain-
Bi0.5 Na0.5 TiO3 (BNT) is considered a promising lead-
orientation [14]. Nevertheless, BNT-based ceramics tex-
free candidate material because of its large polarization
tured by ST templates exhibit a high degree of grain
and high Currie temperature. In the last decades, BNT
orientation, however, the paraelectric phase of ST ceramics is believed to decrease depolarization tempera-
∗ E-mail:
[email protected]
ture and have adverse effects on the electromechanical
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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properties [11]. Beside this, a large amount of plate-like
was washed from the product and the by-product of bis-
BIT templates is usually required to produce highly tex-
muth oxide (Bi2 O3 ) was eliminated by reacting with HCl
tured ceramics [15], which makes the ceramics difficult
solution [19]. Finally, the as synthesized BNT templates
to densify. If similar structure, plate-like BNT templates
were pressed into pallets and sintered at 1150 ◦ C for 2 h.
are utilized for fabrication of BNT-based textured ceram-
Crystalline phase and purity of the as synthesized
ics then it is expected that it will improve the degree of
BNT templates were examined at room temperature
grain orientation and will not change the depolarization
by X-ray diffraction machine (XRD, RAD III, Rigaku, Japan) using CuKα radiation (λ = 1.541 ˚ A). The parti-
temperature. BNT has a simple perovskite structure which possesses
cle morphology and size were examined with a field emis-
a high crystal symmetry. It is very hard to produce
sion scanning electron microscope (FE-SEM, JP/JSM
plate-like large anisotropic BNT templates by conven-
5200, Japan). The bulk density of the sintered BNT was
tional procedures, such as mixed oxide route [16], sol-gel
measured through Archimedes method and was found to
[17] or hydrothermal methods [18]. The layered structure
have 91% of its theoretical value. For electrical measure-
ferroelectric materials such as Bi4.5 Na0.5 Ti4 O15 (BNT4),
ments, the upper and the lower surfaces of the speci-
+
where the A-site is co-occupied by Na
3+
and Bi
has
men were polished to become parallel and coated with a
been experimentally verified to form plate-like templates
silver-palladium paste as electrode by screen printing.
through molten-salt processes. The resemblance of the
The dielectric constant and loss responses were mea-
crystal structures of the layered structure BNT4 and sim-
sured through an impedance analyzer (HP4194A, Agi-
ple BNT facilitates the transformation from the layer-
lent Technologies, Palo Alto, CA). Polarization versus
structured BNT4 into a simple perovskite BNT by a
electric field (P − E) hysteresis loops were measured in
topochemical reaction.
Here we report, the prepara-
silicone oil with the aid of ferroelectric test system (Pre-
tion of plate-like BNT templates from layered structure
cision LC; Radian Technologies, Albuquerque, NM) at
BNT4 via a topochemical method and investigation of
20 Hz.
their structural, dielectric and ferroelectric properties.
III. RESULTS AND DISCUSSION II. EXPERIMENTAL PROCEDURE Fig. 1 shows the XRD pattern and FE-SEM microThe starting materials, reagent-grade Bi2 O3 , TiO2
graph of BNT4 templates prepared by molten salt syn-
and Na2 CO3 powders of purity more than (99.9%) were
thesis (MSS). The XRD profile indicates a single phase
mixed according to the stoichiometric formula of BNT4.
without any evidence of unwanted phases with all diffrac-
Sodium chloride (NaCl) salt (99.95%) was mixed with
tion peaks attributable to a layered perovskite structure.
them in a salt to oxide weight ratio 1.5:1, followed by
All the diffraction matches well with the JCPDS card
ball-milling in ethyl alcohol for 24 h. After removal of
no. 74-1316. Majority of the peaks such as (006), (008),
balls and drying, the dried powder was put in a tightly
(0010), (0016), and (0020) shows higher intensities than
◦
covered Al2 O3 crucible and heat treated at 1100 C for 4
other peaks (101), (107), (109), (110) etc., suggesting
h. After the completion of reaction, NaCl was removed
that the surface of BNT4 templates are parallel to (00l)
from the as-synthesized product by washing thoroughly
plane and a high degree of grain orientation for BNT4
with hot de-ionized water. BNT4 platelets, Na2 CO3 and
templates [19]. The FE-SEM micrograph of BNT4 shows
TiO2 were then weighed to give a total composition of
that all templates have plate-like morphology with an av-
BNT. NaCl salt was again mixed to them with same
erage grain size (10 - 15 µm). Some submicron size grains
weight ratios followed by mixing for 5 h in ethanol solu-
were also observed, which could be the broken pieces of
tion with magnetic stirrer. The slurry was dried and heat
BNT4 crystals. BNT4 belongs to BLSF family which
◦
treated at 950 C for 4 h in a tightly covered alumina
is highly anisotropic with growth along a(b) axis much
crucible. Subsequently, with hot de-ionized water, NaCl
higher than c-axis. Therefore, it reasonable for them to
Structural and Electrical Properties of Bi0.5 Na0.5 TiO3 Templates Produced by · · · – Ali Hussin et al.
717
Fig. 1. (a) XRD patterns and (b) FE-SEM micrograph of Na0.5 Bi4.5 Ti4 O15 precursor particles produced by molten salt synthesis at 1100 ◦ C for 4 h.
Fig. 2. (a) XRD patterns and (b) FE-SEM micrograph of Bi0.5 Na0.5 TiO3 templates produced from Bi4.5 Na0.5 Ti4 O15 precursor.
form plate-like morphology during appropriate processing. These as synthesized BNT4 templates were used as precursor materials for topochemical microcrystal conversion (TMC) process in this work. The XRD pattern along with the FE-SEM micrograph of BNT templates produced from the BNT4 using TMC method was recorded and is shown in Fig. 2. BNT templates produced by TMC method shows a singlephase perovskite structure, which matches well with the JCPDS card No. 36-0340 of the Na0.5 Bi0.5 TiO3 ceramics. Because of a small rhombohedral distortion, all the intensity peaks were indexed on the basis of pseudocubic perovskite unit cell. The XRD profile provides clear information that after the TMC process, the layerstructured BNT4 templates have been completely transformed into a simple perovskite BNT templates, which reserved the parent plate-like morphology. Most of the peaks in plate-like BNT templates laid down with the caxis aligning along the vertical direction during the sample synthesis. So, they exhibit strong (100) and (200) diffraction peaks [19]. Fig. 2(b) shows the FE-SEM micrographs of the BNT templates synthesized from the
BNT4 templates through TMC method. Analogous to the BNT4 templates, most of the BNT templates have plate-like morphology and large grain size. Such types of large and plate-like particles are quite suitable for producing textured ceramics by the tape-casting process. BNT4 belongs to the family BLSFs, which contain (Bi2.5 Na0.5 Ti4 O13 )2− (pseudo-) perovskite layers enclosed in (Bi2 O2 )2+ fluorite layers, where the A site is co-occupied by Bi3+ and Na+ in a Na/Bi ratio of 0.2. This conversion from the layered structure to the simple perovskite is comprised of two processes: first, the diffusion of Na+ and Bi3+ in the perovskite layers, while second is the change of the (Bi2 O2 )2+ fluorite layers to the perovskite structure. It has been also reported that this transformation is from a lamellar phase to a perovskite phase [20], the process involving the (Bi2 O2 )2+ layers changing to the perovskite structure still need further verifications. Fig. 3(a), and Fig. 3(b) shows XRD profile and FESEM micrograph of the sintered BNT templates produced through TMC process.
Within the detection
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New Physics: Sae Mulli, Vol. 65, No. 8, August 2015
Fig. 4. (Color online) Temperature dependent dielectric response of Bi0.5 Na0.5 TiO3 ceramics produced by TMC method. The temperature dependence of the dielectric constant and loss of sintered BNT templates with different frequencies (1 - 100 kHz) are shown in Fig. 4. Two dielectric anomalies, commonly known as the depolarization temperature (Td ) and permittivity maximum temperature (Tm ) are appeared at temperature 195 ◦ C and 340 Fig. 3. (a) XRD patterns and (b) FE-SEM micrograph of Bi0.5 Na0.5 TiO3 ceramics sintered at 1150 ◦ C for 2 h in air atmosphere.
◦
C, respectively, These two anomalies are consistent with
the dielectric behavior of other BNT and BNT-based ceramics [16, 21]. Both Td and Tm anomalies are broad-
limit of XRD, the pattern shows a single phase perovskite structure without any traces of unwanted parasite phases. This suggest that BNT produced by TMC form a complete solid solution after sintering. Close inspection of the XRD pattern disclosed no splitting of the characteristic peaks at 2θ angle 40◦ and 46◦ and all peaks matches well with the standard data of pseudocubic perovskite unit cell. This study also shows that TMC process does not bring an obvious change in the crystal structure of BNT templates. The FE-SEM picture of BNT indicates a dense and compact microstructure. Small and big grains are homogeneously distributed with some pores. Sintering has a significant effect on the grain growth and grain morphology of BNT templates. Overall, the grain morphology of the sample is changed from plate like to spherical shape having a polycrystalline nature. Before sintering, polyvinyl alcohol (PVA) binder was mixed with the plate-like BNT templates, crushed and then pressed into pellet at a pressure of 50 MPa. The crushing and pressing are expected to break the platelike morphology, where the subsequent heat treatment results in equaxial grain morphology.
ened with increasing frequency. Furthermore, the value of dielectric constant at Tm decrease with increasing frequency, however, no significant change in the Tm temperature can be observed. In addition, the Td temperature increases with increasing frequency suggesting relaxor like behavior of the sample. The sample exhibits low dielectric loss at room temperature, however, this loss increase at high temperatures 400 ◦ C and above. The high value of the dielectric loss at higher temperature may be due to transport of ions with higher thermal energy. The rapid increase in the dielectric loss curves beyond the 400 ◦ C may be due to the scattering of the thermally activated charged carriers and some defects in the samples. At elevated temperatures the conductivity begins to dominate which in turn is responsible for the rise in dielectric loss (since σ ∝ tan δ) [22]. It is well known that the hysteresis loops is a useful probe to investigate the ferroelectric behavior of the sample. The measurement of P − E was conducted to investigate the ferroelectric properties of the sintered BNT templates. Fig. 5 shows the room temperature P − E
Structural and Electrical Properties of Bi0.5 Na0.5 TiO3 Templates Produced by · · · – Ali Hussin et al.
719
and frequency dispersion response of the sample. P − E hysteresis loops show well saturated curve at an applied electric field of 90 kV/cm with a large remanent polarization 32 µC/cm2 and a coercive field of 56 kV/cm. Our results reveal that plate-like BNT templates prepared by TMC method are suitable for the development of high performance BNT-based textured ceramics.
ACKNOWLEDGEMENTS Fig. 5. (Color online) P − E hysteresis loops of Bi0.5 Na0.5 TiO3 ceramics produced by TMC method measured at different fields.
This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Education (MOE) (2013R1A1A2058345) and Basic Research program
hysteresis loops of the BNT ceramics at a measuring frequency of 50 Hz under different fields (70 - 90 kV/cm). It is interesting to note that the loops show a typical ferroelectric behavior at all fields, which is characterized by definite squareness in the P − E hysteresis loop with a certain remanent polarization (Pr ) and a coercive field (Ec ). However, at 70 kV/cm the loop is not well saturated. When the field is increased from 70 to 90 kV/cm, polarization as well as coercive field increase and the loops become well saturated. At 90 kV/cm, high remanent polarization 32 µC/cm2 and a coercive field (Ec ) of 56 kV/cm can be observed, which show successful synthesis of BNT templates by TMC method. The well-defined dielectric and ferroelectric response of BNT along with its plate-like morphology and uniform distribution are expected to act as a substrate for epitaxy and as a seed in the matrix powder of BNT-based ceramics to develop high performance textured ceramics by reactive template grain growth method.
through the National Research Foundation of Korea (NRF) funded by Ministry, Science and Technology (MEST) (2011-0030058).
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